Abstract: Effect of prestrain on the bake hardening value and microstructure of high aluminum dual phase steel HC420/780DP was studied. After prestraining the high aluminum dual phase steel HC420/780DP at 0%, 2%, 4%, 6%, 8% and 12% respectively, and then bake hardened at 170 ℃ for 20 min, the bake hardening value was measured, and the specimens with different prestrains were detected and analyzed by means of OM, SEM, XRD and other analysis methods. The results indicate that the bake hardening value is directly related to the density of dislocations. As the prestrain increases, the dislocation density continuously increases, the martensite content increases, and the bake hardening value also increases. When the prestrain is ≤2%, the dislocation density is small, and the bake hardening value is less than 30 MPa. When the prestrain reaches 8%, the dislocation density is the largest, about 0.208 m^-2, and the bake hardening value reaches 38 MPa. When the prestrain is ≥8%, the dislocation density remains unchanged, but the bake hardening value continues to increase, reaching a maximum of 72 MPa. The XRD results show that no retained austenite is found before and after baking under different prestrains.

Key words: cold rolled dual phase steel, bake hardening, prestrain, Cottrell atmosphere